Raw blank for vacuum carburization and method for producing same

ABSTRACT

A raw blank for vacuum carburization is provided. The raw blank for vacuum carburization has a chemical composition containing, by mass, C: 0.13 to 0.28%, Si: 0.01 to 1.20%, Mn: 0.10 to 1.50%, P: 0.030% or less, S: 0.050% or less, Cr: 0.30 to 2.20%, Al: 0.027 to 0.090%, N: 0.0060 to less than 0.0140%, and, as an optional element, Mo: 0.60% or less, the remainder being Fe and unavoidable impurities, and satisfying the following formula (1). AlN precipitates having an equivalent circle diameter of 100 nm or more exist at 1.5 pieces/100 μm2 or less in a cross section of the raw blank.Al×N≤0.00090  (1)It is noted that element symbols in the formula (1) denote a value of content (% by mass) of each element.

TECHNICAL FIELD

The present invention relates to a raw blank for vacuum carburization and a method for producing the same.

BACKGROUND ART

Members, such as gears, that need to surely have high surface hardness are typically formed by forging or the like using a steel material of low carbon alloy steel such as SCM420, and then subjected to a surface hardening treatment by carburizing-quenching-tempering. As a method of carburizing and quenching, gas carburization has been widely used in the past. However, in recent years, vacuum carburization (reduced pressure carburization) is increasingly used in responding to the need for shortening a treatment time and downsizing a lot to be treated. In the vacuum carburization treatment temperature can be set higher than that in gas carburization, therefore, the treatment time of vacuum carburization can be shortened as compared with that of gas carburization. For example, Patent Documents 1 and 2 describe a technique for performing vacuum carburization in the past.

PRIOR ART LITERATURE Patent Documents

Patent Document 1: JP-A-2008-069436

Patent Document 2: JP-A-2014-208867

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

In association with the higher treatment temperature during vacuum carburizing, there problematically occurs abnormal grain growth on a surface portion of a treated product. Abnormal grain growth causes not only reduction of bending fatigue strength, surface fatigue strength, and so on which are structural strength, but also decrease in productivity due to distortion (deformation) increased during quenching, which has to be curtailed.

In order to inhibit abnormal grain growth of the treated surface portion, there is a method in which precipitates of AlN or the like are finely dispersed so that movement of grain boundaries is inhibited when abnormal growth of crystal grains occurs. However, it has been reported that because denitrification (decrease in nitrogen) is more likely to occur on the treated surface portion as compared to the inside portion during vacuum carburization, whereby an amount of AlN precipitated may be reduced, abnormal grain growth may not be inhibited in some cases.

On the other hand, it is not sure that a measure to sufficiently maintain the precipitation amount of AlN by inhibiting denitrification has been satisfactorily established. For example, in Patent Document 1, a reference to occurrence of denitrification can be found, but a disclosure of a countermeasure against the denitrification cannot be found. Further, Patent Literature 2 includes a proposal for a method for preventing denitrification, a countermeasure on the premise of supplying a nitriding gas such as ammonia during a carburization treatment, but this countermeasure not only adversely affects the durability of treatment facilities but also causes an increase in production cost. Therefore, it is difficult to actually adopt the countermeasure.

The present invention has been made in view of such a background, and an object of the present invention is to provide a raw blank for vacuum carburization in which abnormal grain growth of a treated product especially on its surface is inhibited during vacuum carburization, and a method for producing the raw blank for vacuum carburization.

Means for Solving the Problems

One aspect of the present invention is a raw blank for vacuum carburization, having:

a chemical composition containing, by mass, C: 0.13 to 0.28%, Si: 0.01 to 1.20%, Mn: 0.10 to 1.50%, P: 0.030% or less, S: 0.050% or less, Cr: 0.30 to 2.20%, Al: 0.027 to 0.090%, N: 0.0060 to less than 0.0140%, and, as an optional element, Mo: 0.60% or less, the remainder being Fe and unavoidable impurities, and satisfying the following formula (1),

wherein AlN precipitates having an equivalent circle diameter of 100 nm or more exist at 1.5 pieces/100 μm² or less in a cross section of the raw blank:

Al×N≤0.00090  (1)

wherein element symbols in the formula (1) denote a value of content (% by mass) of each element.

Another aspect of the present invention is a method for producing the raw blank for vacuum carburization, the method including:

heating a steel material having the chemical composition to a temperature of 1100° C. or higher to perform final hot working; and

thereafter cooling the steel material subjected to the final hot working to 900° C. at a cooling rate of 1° C./sec or more.

Effects of the Invention

In the chemical composition of the raw blank for vacuum carburization, first, a nitrogen (N) content rate is controlled to a lower level than that in the past for countermeasure to denitrification. Specifically, because a larger amount of nitrogen contained in steel facilitates denitrification, the denitrification is inhibited by preliminarily lowering the amount of nitrogen to be contained in steel.

In addition, because if the nitrogen content merely is set lower, the precipitation amount of AlN, which is effective for inhibiting abnormal grain growth, is reduced, therefore a measure for setting an addition amount of Al higher than that in the past is simultaneously taken while the minimum nitrogen content rate is surely maintained. On the other hand, if the addition amount of Al is excessively increased, coarse AlN is likely to be generated. Thus, by satisfying the relationship of Formula (1), the addition amount of Al is controlled depending on an addition amount of N.

In addition, as the result of the experiments conducted by the present inventors under various conditions, the following findings have been obtained. That is, the present inventors have found that, when coarse AlN exists before the AlN precipitation treatment, it becomes easy for the coarse AlN to preferentially continue to grow and coarsen, in the meantime, it become difficult that new fine AlN is precipitated, and crystal grains are easily coarsened accordingly. Then, the present inventors have made further studies and consequently have found the fact that when the production method is devised so that AlN is sufficiently dissolved in a solid form at the time of producing a raw blank, and at the same time, the coarse AlN precipitates having an equivalent circle diameter of 100 nm or more exist at 1.5 pieces/100 μm² or less in a cross section of the raw blank, abnormal grain growth on the treated surface portion during vacuum carburization can be reliably curtailed under appropriate treatment conditions which will be described later, and have clarified its production conditions.

Specifically, in the above production method, when the final hot working is performed, the steel material is heated to a temperature of 1100° C. or higher to perform the hot working, and then is cooled to 900° C. at a cooling rate of 1° C./sec or more. In this method, generation of coarse AlN is curtailed by sufficiently dissolving AlN in a solid form at the time of the hot working, and then performing cooling to 900° C. at a high cooling rate of 1° C./sec or more. As a result, the requirement that coarse AlN precipitates having an equivalent circle diameter of 100 nm or more exist at 1.5 pieces/100 μm² or less in the cross section of the raw blank for vacuum carburization can be achieved.

In this way, a raw blank for vacuum carburization which can reliably curtail abnormal grain growth on the treated surface portion during vacuum carburization can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an SEM image for observation of AlN in Example 4.

FIG. 2 is an SEM image for observation of AlN in Comparative Example 18.

MODES FOR CARRYING OUT THE INVENTION

First, the reason for limiting the chemical composition of the raw blank for vacuum carburization will be described.

C: 0.13 to 0.28%

C (carbon) is an element necessary for improving hardness after a quenching treatment and obtaining internal hardness for secured strength. In order to obtain this effect, C is contained in an amount of 0.13% or more. On the other hand, excessive addition of C leads to an excessive increase in hardness before machining and a decrease in workability, and thus an upper limit of a C content rate is set to 0.28% in order to prevent such an excessive increase in hardness and such a decrease in workability.

Si: 0.01 to 1.20%

Si (silicon) is an element that is essential as a deoxidizer during steelmaking, and is also an element that suppresses generation of carbides at the time of tempering and improves temper softening resistance. In particular, in order to obtain an effect as the deoxidizer, Si is contained in an amount of 0.01% or more. On the other hand, excessive addition of Si leads to an excessive increase in hardness before machining and a decrease in workability, and thus an upper limit of an Si content rate is set to 1.20% in order to prevent such an excessive increase in hardness and such a decrease in workability.

Mn: 0.10 to 1.50%

Mn (manganese) is an element that acts as a deoxidizer during steelmaking, and is also an element effective for improving hardenability. In order to obtain this effect, Mn is contained in an amount of 0.10% or more. On the other hand, excessive addition of Mn leads to an excessive increase in hardness before machining and a decrease in workability, and thus an upper limit of an Mn content rate is set to 1.50% in order to prevent such an excessive increase in hardness and such a decrease in workability.

P: 0.030% or less

P (phosphorus) is an element contained as an impurity. It is an element that is likely to segregate at austenite grain boundaries, and is also an element that, when segregated, causes a decrease in bending fatigue strength. Therefore, an upper limit of a permissible content rate of P is set to 0.030%.

S: 0.050% or less

S (sulfur) is an element contained as an impurity. In addition, S is well known as an element that improves machinability. However, when S is contained in a large amount, a sulfide-based nonmetallic inclusion increases, which causes a decrease in fatigue strength. Therefore, an upper limit of a permissible content rate of S is set to 0.050%.

Cr: 0.30 to 2.20%

Cr (chromium) is an element that enhances hardenability. In order to obtain this effect, Cr is contained in an amount of 0.30% or more. On the other hand, excessive addition of Cr leads to an excessive increase in hardness before machining and a decrease in workability, and thus an upper limit of a Cr content rate is set to 2.20% in order to prevent such an excessive increase in hardness and such a decrease in workability.

Al: 0.027 to 0.090%

Al (aluminum) is an element used as a deoxidizer during steelmaking, and exhibits an effect of suppressing abnormal grain growth at the time of carburization when Al binds to N and exists as fine AlN. In order to precipitate AlN which is necessary for suppressing abnormal grain growth on the treated surface portion, it is necessary to set an Al content rate to 0.027% or more. On the other hand, excessive addition of Al causes generation of coarse AlN or failure to sufficiently dissolve AlN in a solid form during hot working (hot rolling or hot forging) before a carburization treatment, so that fine AlN may not be sufficiently precipitated at the time of subsequent vacuum carburization temperature rise, and abnormal grain growth may not be suppressed. Therefore, an upper limit of the Al content rate is set to 0.090%.

N: 0.0060 to less than 0.0140%

N (nitrogen) is an element that exhibits an effect of curtailing abnormal grain growth during carburization when N exists as AlN in combination with Al. In order to precipitate AlN which is necessary to curtail abnormal grain growth, it is necessary to set an N content to 0.0060% or more. On the other hand, excessive addition of N increases a heating temperature during hot working to secure sufficient dissolving of the precipitated AlN in a solid solution, which may lead to an increase in production cost and a decrease in facility life, and thus an upper limit of the N content rate is set to less than 0.0140%. Here, it is noted that because the N content rate is controlled to be relatively low as described above, the amount of AlN precipitates is at the level considered not to be so large in comparison with that of a case hardening steel for carburization in the past. Therefore, crystal grain coarsening may occur not only on the treated surface portion but also inside, and thus it is necessary to set a carburization treatment temperature to curtail the crystal grain coarsening. This will be described later.

Mo as optional element: 0.60% or less (including the case of 0%),

Mo (molybdenum) is an element that has an effect of enhancing hardenability and improves temper softening resistance, and thus it can be added when needed as an optional additive element. On the other hand, when an excessive amount of Mo is contained, hardness before machining excessively increases, which causes decrease in workability and cost increase. Therefore, in order to prevent such problems, an upper limit of Mo content within a permissible range is set to 0.60%.

Al×N≤0.00090  (1)

The content rates of Al and N need to be limited so that each element meets the specified range and at the same time satisfies Formula (1). This makes it possible to curtail generation of coarse AlN before carburization.

In addition, the AlN precipitates having an equivalent circle diameter of 100 nm or more in the cross section of the raw blank for vacuum carburization needs to exist at 1.5 pieces/100 μm² or less. That is, the number of coarse AlN precipitates having an equivalent circle diameter of 100 nm or more needs to be reduced to at least 1.5 pieces/100 μm² or less. As a result, it is possible to achieve a state where fine AlN is sufficiently precipitated while maintaining a state where coarse AlN is not present or, even if present, present in a very small amount in a temperature raising process during vacuum carburization, and it is possible to curtail abnormal grain growth at the time of vacuum carburization along with the denitrification suppressing effect due to the N content rate being limited to a relatively low value.

In addition, the raw blank for vacuum carburization is preferably a ferrite-pearlite structure hardly containing bainite in the internal structure. This not only can improve cutting workability, but also can reduce grain growth driving force during vacuum carburization and suppress abnormal grain growth.

Next, in order to obtain an excellent raw blank for vacuum carburization as described above, the following production method can be adopted. Specifically, a method for producing the raw blank for vacuum carburization, the method including: when a steel material having the chemical composition is subjected to final hot working, heating the steel material to a temperature of 1100° C. or higher to perform the hot working, and then cooling the hot-worked steel material to 900° C. at a cooling rate of 1° C./sec or more, can be applied.

The final hot working mentioned above is defined as follows. In the case of performing hot working only once, this hot working corresponds to the final hot working. Meanwhile, in the case of performing hot working multiple times, the hot working performed last time corresponds to the final hot working. As the final hot working, hot forging is typical, however, hot rolling or any other hot plastic working method is available.

In the above production method, the final hot working is performed after the steel material is heated to a temperature of 1100° C. or higher. By heating at the above temperature during the hot working, coarse AlN present in the steel material can be dissolved, and the number of coarse AlN can be reduced so as to fall within the range of the condition described above. An excessively high heating temperature during the hot working is not preferred from the viewpoint of energy loss and productivity reduction, and an upper limit of the heating temperature is preferably 1260° C.

Next, a cooling condition for a raw blank, which is formed from the steel material by the final hot working to have a desired shape, is set to be relatively rapid such that cooling to 900° C. is performed at a cooling rate of 1° C./sec or more (preferably, 1.5° C./sec or more). In particular, in the case of producing a relatively large-sized component, this condition cannot be satisfied by naturally cooling the blank left in the air, and thus, in order to purposely increase the cooling rate, any control, for example, by fan cooling is required. Such control makes it possible to curtail generation of coarse AlN after hot working, and to reliably achieve the above requirement of AlN in the raw blank for vacuum carburization.

In one specific production method for producing the above-described raw blank for vacuum carburization, after performing melting and component adjustment on a raw material, casting is performed thereon to form an ingot, thereafter rough processing such as hot rolling, and then hot forging, hot rolling, or the like as the final hot working described above are performed on the ingot.

It is noted that after the final hot working, annealing may be additionally performed. Addition of the annealing process assures enhanced mechanical workability to shape a final component.

Annealing has been a well-known heat treatment as such, and can be performed under various well-known conditions. For example, the annealing can be performed under the condition that the raw blank for vacuum carburization, which has been cooled to 900° C. under the above-mentioned cooling condition and thereafter further cooled to around room temperature after the final hot working, is heated to a temperature range of 850° C. to 900° C., then slowly cooled to around 600° C. to 700° C., and then allowed to stand to cool to room temperature.

Annealing can also be performed by utilizing the heat applied during the final hot working. Specifically, there can be adopted an annealing condition that the raw blank, which has been cooled to 900° C. under the above-mentioned cooling condition, is held in the temperature range of 600° C. to 680° C. for 40 minutes to 120 minutes in the process of further cooling, and then allowed to stand to cool to room temperature.

The raw blank for vacuum carburization thus obtained is typically subjected to vacuum carburization after cutting work has been performed thereon, and then subjected to finishing work. Here, in some cases, the vacuum carburization is also be referred to as reduced pressure carburization. For example, the vacuum carburization is performed in a treatment furnace that maintains a treatment temperature while introducing a carburization gas such as acetylene in pulse state thereinto under the condition that an ambient pressure in the furnace is reduced to lower than atmospheric pressure. The pressure reduction condition is preferably within the range of 50 to 3000 Pa. As the carburization gas, for example, a hydrocarbon gas, particularly, acetylene or the like can be used.

Here, the treatment temperature in the vacuum carburization is preferably set to be 980° C. or higher from the viewpoint of productivity improvement, and is preferably lower than T1 (° C.) represented by the following formula (2) and lower than T2 (° C.) represented by the following formula (3):

T1=300×√(Al−0.027)+1000  (2)

T2=160000×Al×N+955  (3)

(It is noted that element symbols in the formulae (2) and (3) each indicate a content (% by mass) value of each element.

In detail, the chemical composition is optimized so that crystal grain coarsening can be curtailed effectively for the N content that is set to be relatively low. However, the upper limit of the treatment temperature at which crystal grain coarsening can be curtailed varies depending on the Al and N contents. Under such condition, a number of experiments for the upper limit temperature have been conducted, so that the experimental formulas have been found, which are the formulas (2) and (3) described above. Of the above formulas, the formula (2) represents the upper limit of the treatment temperature at which crystal grain coarsening due to denitrification does not occur on the treated surface portion, and Formula (3) represents the upper limit of the treatment temperature at which crystal grain coarsening does not occur in the entire material to be treated including not only the treated surface portion but also the inside portion.

It is noted that in the chemical composition, the N content rate is controlled to be relatively low in consideration of the influence of denitrification during vacuum carburization. As a result, the amount of resulted AlN precipitates is at the level considered not to be so large in comparison with that of a hardening steel for carburization in the past. Thus, also in regard to the crystal grain coarsening that occurs entirely in the material to be treated, it has been figured out that the limit of the treatment temperature at which mixed grains can be prevented from being generated varies depending on the content rates of Al and N. Accordingly, the formula to determine the limit treatment temperature has been obtained through a number of experiments as the formula (3).

EXAMPLES Experimental Example 1

Examples of the raw blank for vacuum carburization and the method for producing the same described above will be described.

In this example, as shown in Table 1, various samples were prepared using samples (Examples 1 to 13, Comparative Examples 14 to 21, and Reference Examples 22 and 23) made of 23 types of steel materials having different chemical components, and evaluated.

TABLE 1 Formula (I) Sample No. C Si Mn P S Cr Mo Al N Fe Al × N Example 1 0.13 0.06 1.46 0.011 0.006 0.98 0.16 0.073 0.0095 bal. 0.00069 2 0.18 0.89 0.60 0.021 0.027 0.42 0.54 0.067 0.0105 bal. 0.00070 3 0.28 0.35 0.39 0.018 0.009 0.73 0.44 0.055 0.0124 bal. 0.00068 4 0.21 0.21 0.75 0.013 0.018 1.52 0.02 0.060 0.0136 bal. 0.00082 5 0.23 0.15 0.82 0.017 0.011 1.14 0.25 0.028 0.0072 bal. 0.00020 6 0.20 0.22 0.68 0.008 0.022 1.21 0.30 0.042 0.0085 bal. 0.00036 7 0.24 0.54 1.20 0.023 0.037 0.56 0.39 0.084 0.0092 bal. 0.00077 8 0.19 1.14 0.44 0.012 0.013 2.12 0.15 0.055 0.0113 bal. 0.00062 9 0.17 0.42 0.54 0.009 0.009 0.34 0.35 0.035 0.0068 bal. 0.00024 10 0.28 0.24 0.72 0.012 0.012 1.03 0.22 0.048 0.0130 bal. 0.00062 11 0.22 0.74 0.52 0.014 0.013 0.60 0.52 0.063 0.0072 bal. 0.00045 12 0.21 0.22 0.74 0.013 0.018 1.47 0.02 0.046 0.0094 bal. 0.00043 13 0.22 0.19 0.77 0.013 0.020 1.60 0.01 0.058 0.0110 bal. 0.00064 Comparative 14 0.21 0.17 0.75 0.012 0.015 1.12 0.18 0.023 0.0100 bal. 0.00023 Example 15 0.18 0.27 0.84 0.008 0.010 0.99 0.02 0.030 0.0055 bal. 0.00017 16 0.20 0.21 0.80 0.014 0.011 1.04 0.21 0.046 0.0148 bal. 0.00068 17 0.25 0.53 0.48 0.021 0.023 0.55 0.49 0.070 0.0138 bal. 0.00097 18 0.16 0.72 0.14 0.016 0.018 0.34 0.15 0.087 0.0128 bal. 0.00111 19 0.21 0.19 0.74 0.023 0.027 1.47 0.01 0.046 0.0102 bal. 0.00047 20 0.22 0.23 0.81 0.011 0.019 1.54 0.02 0.038 0.0075 bal. 0.00029 21 0.19 0.31 0.67 0.024 0.031 1.20 0.24 0.057 0.0116 bal. 0.00066 Reference 22 0.24 1.00 0.48 0.016 0.013 1.97 0.20 0.065 0.0083 bal. 0.00054 Example 23 0.22 0.20 0.77 0.012 0.019 1.08 0.17 0.027 0.0137 bal. 0.00037

Each steel material prepared by casting with electric furnace melting was subjected to extend forging to form a steel bar having a diameter φ of 15 mm. This steel bar was subjected to machining to prepare a test piece having a diameter of 8 mm and a height (longitudinal direction) of 12 mm.

To each test piece, working corresponding to the final hot working was applied. Specifically, the test piece was heated at a heating temperature shown in Table 2 using “Thermec Master” manufactured by Fuji Electronic Industrial Co., Ltd., and then subjected to upset forging at a compression rate of 25%. After the upset forging, the test piece was cooled to 900° C. at a cooling rate shown in Table 2, and then subjected to an annealing treatment by either one of two methods described below.

In the annealing treatment indicated as “IA” in Table 2, the test piece is once cooled to room temperature, then heated to 900° C., held at the temperature for 60 minutes while being heated, then slowly cooled to 600° C., and allowed to stand to cool to room temperature.

In the annealing treatment indicated as “FIA” in Table 2, the test piece is cooled to 900° C. at a cooling rate shown in Table 2 after the final hot working, then allowed to continuously stand to cool until a surface temperature of the test piece reaches 650° C., held at 650° C. for 60 minutes while being heated, and then allowed to stand to cool to room temperature.

On the test piece having the annealing treatment performed thereon, which had not been subjected to a vacuum carburization treatment, metallographic structure observation was performed to confirm a structure state and calculate the equivalent circle diameter and number density of precipitated AlN. The results are shown in Table 2. In Table 2, the indication “F+P” means a ferrite/pearlite structure.

An AlN precipitation state was observed in the following manner. That is, the annealed test piece was cut to make a surface perpendicular to the longitudinal direction of the test piece, and subjected to embedded polishing. The polished surface was etched and observed by FE-SEM (field emission scanning electron microscope). For measurement, 10 visual fields were observed at a visual field of 20,000 magnification, and an SEM image was taken. The SEM image was subjected to image analysis using image analysis software “Quick GrainStandard ” to calculate the equivalent circle diameter and number density of AlN. In order to identify the precipitates as AlN, EDX (energy dispersive X-ray analysis) was also performed.

In FIGS. 1 and 2 , examples of SEM images in Example 4 and Comparative Example 18, respectively are shown for reference. In Example 4 shown in FIG. 1 , no coarse AlN precipitate was observed in the SEM image. In Comparative Example 18 shown in FIG. 2 , at least two AlNs (particles indicated by white arrows) were confirmed in one visual field shown in FIG. 2 in the SEM image, and all the equivalent circle diameters were 100 nm or more, that is, about 190 nm (upper part of FIG. 2 ) and about 150 nm (lower part of FIG. 2 ).

Next, on the test piece after subjected to the annealing treatment, a vacuum carburization treatment was performed at a temperature shown in Table 2. A pressure in the furnace was controlled to be 100 Pa, acetylene (C₂H₂) was used as a carburization gas, and a carburization time was set to 1.5 h. Further, in Table 2, the upper limit temperature calculated according to Formula (2) and the upper limit temperature calculated according to Formula (3) are shown as the carburization treatment temperatures to be regulated as upper limits. The calculation results obtained from Formulas (2) and (3) are effective only when the chemical composition falls within the appropriate range described above.

On each test piece after subjected to vacuum carburization, metallographic structure observation was performed. Specifically, the test piece was cut to make a surface passing through the center of the test piece and parallel to the longitudinal direction of the test piece and the surface was etched with an aqueous picric acid solution, and then arbitrary 10 visual fields were observed at 100 magnification using an optical microscope. In the case where a region in which grain growth can be found in terms of increase in grain size number by three or more as compared to the other region accounts for 20% or more of the observed range, the test piece was determined to have “mixed grains” and to exhibit abnormal grain growth. It is noted that observation targets not only a surface layer portion of the test piece for which an influence of denitrification should be recognized but also the entire test piece. Measurements of crystal grain size were performed by the method completely according to JIS G0551 standard.

TABLE 2 Existence or Nonexistence Conditions of Number of of Abnormal Final Hot Working Precipitated Grain Growth Heating Cooling Category Coarse AIN Calculated Calculated Actual after Vacuum Temperature Rate of Internal (pieces/100 Value of Value of Treatment Carburization Sample No. (° C) (° C/sec) Annealing Structure μm²) Formula (2) Formula (3) Temperature Temperature Example 1 1170 1.5 IA F + P 0.0 1064 1066 1020 Nonexistence 2 1230 2.0 FIA 0.0 1060 1068 1050 Nonexistence 3 1200 1.0 FIA 1.1 1050 1064 1000 Nonexistence 4 1200 2.0 FIA 0.0 1054 1086 1020 Nonexistence 5 1150 1.7 FIA 0.0 1008 987 980 Nonexistence 6 1170 2.0 FIA 0.0 1037 1012 1010 Nonexistence 7 1250 1.3 IA 0.3 1072 1079 1040 Nonexistence 8 1170 2.5 IA 0.0 1050 1054 1020 Nonexistence 9 1130 1.5 IA 0.3 1027 993 990 Nonexistence 10 1170 1.0 FIA 1.4 1043 1055 1000 Nonexistence 11 1150 1.7 IA 0.0 1057 1028 1000 Nonexistence 12 1170 1.3 FIA 0.1 1041 1024 1010 Nonexistence 13 1220 1.7 IA 0.0 1053 1057 1040 Nonexistence Comparative 14 1150 2.0 FIA F + P 0.0 — 992 990 Existence Example 15 1120 1.3 IA 0.0 1016 981 980 Existence 16 1150 1.7 FIA 1.7 1041 1064 1010 Existence 17 1180 1.7 IA 2.2 1062 1110 1040 Existence 18 1200 2.0 FIA 3.0 1073 1133 1060 Existence 19 1150 0.3 FIA 6.6 1041 1030 1010 Existence 20 1130 0.8 FIA 1.6 1031 1001 1000 Existence 21 1200 0.6 IA 2.0 1052 1061 1050 Existence Reference 22 1230 1.7 IA 0.0 1058 1041 1050 Existence Example 23 1200 1.7 IA 0.0 1000 1014 1010 Existence

As shown in Tables 1 and 2, Examples 1 to 13 each have a chemical composition falling within the appropriate range and satisfying Formula (1). In these examples, under the condition that the vacuum carburization had not been performed on the test pieces of a raw blank for vacuum carburization treatment, coarse AlN precipitates having an equivalent circle diameter of 100 nm or more were found in the test pieces at 1.5 pieces/100 μm² or less. Even after the vacuum carburization treatment had been performed on the test pieces, no abnormal grain growth was found in the entire test pieces.

On the other hand, regarding Comparative Example 14 corresponding to JIS steel SCM420, it is considered that because of too low an Al content rate and too small an amount of fine AlN precipitates, abnormal grain growth occurred.

Regarding Comparative Example 15, it is considered that because of too low an N content rate and too small an amount of fine AlN precipitates, abnormal grain growth occurred.

Regarding Comparative Example 16, it is considered that because an N content rate was too high and coarse AlN precipitates having an equivalent circle diameter of 100 nm or more existed at more than 1.5 pieces/100 μm², so that abnormal grain growth occurred.

Regarding Comparative Examples 17 and 18, it is considered that because a combination of the Al and N content rates did not satisfy Formula (1), coarse AlN precipitates having an equivalent circle diameter of 100 nm or more existed at more than 1.5 pieces/100 μm², so that abnormal grain growth occurred.

Regarding Comparative Examples 19 to 21, it is considered that because the cooling rate to 900° C. after the final hot working was too slow although the chemical composition was appropriate, coarse AlN precipitates having an equivalent circle diameter of 100 nm or more existed at more than 1.5 pieces/100 μm², so that abnormal grain growth occurred.

Regarding Reference Example 22, it is considered that because the chemical composition and production method were appropriate, an excellent raw blank for vacuum carburization was obtained, but because the treatment temperature of the subsequent vacuum carburization treatment exceeded the calculation result of Formula (3), the occurrence of abnormal grain growth could not be prevented.

Regarding Reference Example 23, it is considered that because the chemical composition and production method were appropriate, an excellent raw blank for vacuum carburization was obtained, but because the treatment temperature of the subsequent vacuum carburization treatment exceeded the calculation result of Formula (2), the occurrence of abnormal grain growth could not be prevented.

Experimental Example 2

In this example, Tests 41 to 44 in which the steel material of Example 4 was used, the heating temperature in the final hot working was set to 1200° C., and the cooling rate to 900° C. after the working was changed were performed to examine the influence on the precipitation state of coarse AlN and the influence on the abnormal grain growth after the vacuum carburization treatment.

TABLE 3 Existence or Nonexistence Number of of Abnormal Precipitated Grain Growth Heating Cooling Category Coarse AIN Calculated Calculated Actual after Vacuum Test Temperature Rate of Internal (pieces/100 Value of Value of Treatment Carburization No. (° C.) (° C./sec) Annealing Structure μm2) Formula (2) Formula (3) Temperature Treatment 41 1200 0.6 FIA F + P 4.3 1054 1086 1020 Existence 42 1.0 FIA 1.1 Nonexistence 43 1.3 FIA 0.2 Nonexistence 44 2.0 FIA 0.0 Nonexistence

As shown in Table 3, in the case where the cooling rate to 900° C. after the final hot working was less than 1° C./sec (Test 41), coarse AlN precipitates having an equivalent circle diameter of 100 nm or more existed at more than 1.5 pieces/100 μm², and abnormal grain growth was observed in the surface layer portion after the vacuum carburization treatment. On the other hand, in the case where the cooling rate to 900° C. after the final hot working was 1° C./sec or more (Tests 42 to 44), coarse AlN precipitates having an equivalent circle diameter of 100 nm or more existed at 1.5 pieces/100 μm² or less, and no abnormal grain growth was observed even after the vacuum carburization treatment. From this result, it is understood that the generation of coarse AlN is effectively curtailed by selecting an appropriate chemical composition, setting the heating temperature of the final hot working to 1100° C. or higher, and appropriately controlling the cooling rate to 900° C. after the working, and thus abnormal grain growth in the vacuum carburization treatment can be curtailed. 

1. A raw blank for vacuum carburization, having: a chemical composition containing, by mass, C: 0.13 to 0.28%, Si: 0.01 to 1.20%, Mn: 0.10 to 1.50%, P: 0.030% or less, S: 0.050% or less, Cr: 0.30 to 2.20%, Al: 0.027 to 0.090%, N: 0.0060 to less than 0.0140%, and, as an optional element, Mo: 0.60% or less, the remainder being Fe and unavoidable impurities, and satisfying the following formula (1), wherein AlN precipitates having an equivalent circle diameter of 100 nm or more exist at 1.5 pieces/100 μm² or less in a cross section of the raw blank: Al×N≤0.00090  (1) wherein element symbols in the formula (1) denote a value of content (% by mass) of each element.
 2. A method for producing the raw blank for vacuum carburization according to claim 1, the method comprising: heating a steel material having the chemical composition to a temperature of 1100° C. or higher to perform final hot working; and thereafter cooling the steel material subjected to the final hot working to 900° C. at a cooling rate of 1° C./sec or more. 